Switched reluctance machines have become increasingly popular in recent years. In a switched reluctance motor, a stator has sets of poles that are sequentially energised to rotate a rotor into line with the energised pair of poles, under the influence of the magnetic fields associated with each set of poles. By rapidly switching between different pairs of poles, it is possible to cause the rotor to rotate at a very high speed.
Recent developments in switched reluctance motors have resulted in higher speeds of rotation of the rotor than was achievable hitherto. However, control of the rotor at such high speeds can be problematic. In particular, the angular position of the rotor at which the poles are energised and de-energised needs to be controlled carefully
It has been proposed to employ control law tables, held in a memory associated with the control circuits of the machine. The control law tables typically comprise look-up tables relating the turn-on and turn-off angles to the speed and torque of the machine over a wide range of operating conditions. However, such control law tables are normally derived by assuming a constant value for the voltage applied to the windings, known as the DC link voltage. In practice, changes in the voltage of the mains power supply, as well as other electrical changes in the environment, result in a DC link voltage that varies over time.
Various proposals have been made to compensate for variance of the applied voltage. For example, in U.S. Pat. No. 6,586,904, it is proposed to sample the DC link voltage, the speed of rotation of the rotor and the torque produced by the motor. These measurements are then used to derive a compensated speed value and a compensated torque value. A look-up table is then employed in order to derive desired operating parameters based on these values.